A pulse combustor operates by producing a series of discrete combustion events rather than a continuous combustion level as is seen in a conventional gas turbine combustion system. These combustions events drive an unstable fluid-dynamic longitudinal mode of vibration, which is evidenced by the pressure in the combustion chamber alternating between high and low pressure. The timing of these combustion events is controlled by the acoustic resonance of the fluid in the combustor, which itself is determined by the geometry of the combustor. The vibration is also evidenced by air in the inlet pipe and tailpipe alternating between forward and reverse flow so that air is periodically ingested and exhausted through both the inlet pipe and tailpipe. A valveless pulse combustor does not comprise mechanical valves. Instead, by virtue of the inlet pipe being substantially shorter than the tailpipe, the air in the inlet pipe offers greater acoustic impedance than the air in the tailpipe. Thus, combustion products are preferentially driven from the combustion chamber to the tailpipe and there is a net flow of air from the inlet pipe to the tailpipe. This is the mechanism by which the valveless pulse combustor self-aspirates.
Since some propulsive force is generated by gas exhaust through the inlet pipe, as well as that generated by the tailpipe exhaust, a mechanism is required to direct the inlet exhaust in a rearward direction. Lockwood-Hiller type combustors use a U-shaped tailpipe and a straight inlet pipe, both pointing rearwardly at their open end. One problem with this arrangement is that there are losses generated by turning the working flow through 180° in the tailpipe.
Kentfield (U.S. Pat. No. 4,033,120) discloses a forward facing inlet pipe and a rearwardly facing tailpipe. It also discloses an inlet-driven ejector that resembles a U-shaped tube with one end coaxial with and spaced apart from the inlet pipe end and the other end approximately parallel to the end of the tailpipe and directed in the same general direction.
One disadvantage of this arrangement is that the combustor is long compared to alternative combustor types. This is particularly disadvantageous for a gas turbine engine application due to the consequent increases in shaft lengths and overall weight.
A further disadvantage of this arrangement is that the first section of the tailpipe, nearest to the combustion chamber, experiences a very high rate of heat transfer and thus tends to get very hot. This problem is exacerbated in a gas turbine engine application since there is generally a shroud, or casing, surrounding the combustor and designed to limit rejection of heat through radiation. Thus, additional cooling may well be required which can cause a substantial penalty in the engine performance.
The present invention seeks to provide a novel valveless pulse combustor that seeks to address the aforementioned problems.